1. Field of the Invention
The present invention relates to a method for determining coordinates of markers contained in a 3D volume data set of a subject provided with markers, with respect to a reference system of coordinates.
2. Description of the Prior Art
In image processing, different procedures employing a 3D volume data set of a measuring subject are associated with the problem of reliably determining the three-dimensional coordinates of the markers contained in the 3D volume data set. Identification of such coordinates is needed in medical technology for example, in order to be able to navigate instruments relative to a patient or in order to be able to handle multi-modality image fusion, namely the superimposition of a number of images acquired by means of different image systems, with the markers as reference points.
U.S. Pat. No. 5,636,255 describes three different methods for determining the positions of markers contained in a data set of a subject provided with markers. In the first method the subject to be examined is disposed in a frame of known dimensions. The frame and the markers arranged at the subject are imaged in the CT data set that is obtained with respect to the subject. Due to the known dimensions of the frame, the positions of the markers imaged in the CT data set can be derived with respect to a reference system of coordinates. The second method for determining the positions of the markers is based on automatically estimating the centers of gravity of the markers in the CT data set, and the third method for determining the positions is based on the utilization of a mechanical pointer.
German OS 195 12 819 describes an X-ray computed tomography device with an X-ray source that emits an X-ray bundle that penetrates a measuring field, and with a detector. A 3D volume data set is to be generated by means of the X-ray computed tomography device; however, the knowledge of the exact pickup geometries with respect to each 2D projection, namely the exact knowledge of the position of the X-radiator and the detector, as well as their orientation relative to one another with respect to each 2D projection, is required. Since the X-ray computed tomography device has mechanical instabilities, markers are arranged in the measuring field, and these markers are imaged in the 2D projections and allow the determination of the pickup geometries for each 2D projection. This document, however, does not describe how the markers, particularly their coordinates, are detected.
German PS 41 20 676 describes a method for detecting small subjects in a natural environment using an electro-optical sensor that scans the surroundings and that is followed by an evaluation unit.
German OS 195 39 367 describes a method for transforming a system of coordinates.
The preferred method for determining the 3D coordinates of the markers in a 3D volume data set is an interactive identification and localization of the markers and therefore is characterized by interventions by a user (attendant, technician, physician, etc.); however, this is time-involved and error-sensitive.
An object of the present invention is to provide a method with which the coordinates of markers contained in a 3D volume data set can be automatically determined in a reliable way, without interventions by a user.
This object is achieved in accordance with the invention in a method for determining the coordinates of markers contained in a 3D volume data set of a subject, which is provided with markers, with respect to a reference system of coordinates, having the following method steps. A series of 2D projections are picked up by means of an X-ray system, which has an X-ray source and a planar X-ray detector, and the 3D volume data set is generated from the series of 2D projections. Alternatively a series of 2D central or 2D parallel projections are generated from an existing 3D volume data set of the subject. The markers imaged in the picked up or generated 2D projections are detected. For each detected marker, one reference point in the 2D projections is determined that represents the detected marker. Back projection straight lines are established that extend through the reference points such that, in the case of the 2D projections picked up by means of the X-ray system, each back projection straight line extends through the focus of the X-ray source. In the case of the generated 2D central projections, each back projection straight line proceeds through the projection point of the respective 2D central projection. In the case of the generated 2D parallel projections, each back projection straight line orthogonally intersects the planar 2D projection in the reference point, which 2D projection contains the imaged marker or the corresponding reference point. The intersection points of the back projection straight lines are identified or the points situated on different back projection straight lines are identified, which points have the smallest distance from one another given back projection straight lines that arc skewed to one another. Spatially limited areas are formed which have an accumulation of intersection points or points having the smallest distance from one another. The coordinates of the center of gravity of each of the spatially limited areas are calculated.
The method makes it possible to fully automatically determine the coordinates of markers contained in a 3D volume data set in a desired way without interventions by a user. Even if, in a number of 2D projections that is comparatively low relative to the total number of the examined 2D projections, contents of the 2D projections are identified as markers in the marker detecting step, these erroneously identified xe2x80x9cmarkersxe2x80x9d are eliminated in the area formatting step, since the number of intersection points of the back projection straight lines and/or the spatial density of the points on the back projection straight line with the smallest distance from one another is too low in order to be combined in a spatially limited area. Conversely, should markers not be recognized as such in a number of 2D projections that is comparatively low relative to the total number of examined 2D projections, these undetected markers are still captured as a spatially limited area in the area forming step due to their recognition in other 2D projections in which the markers are imaged as well. This is because a sufficient number of intersection points of the back projection straight lines and/or of points lying on different back projection straight lines with smallest distance from one another occurs. These points being correspondingly close to one another. Therefore, the inventive method proceeds fully automatically not only when the coordinates of markers contained in a 3D volume data set are determined, but also works in a reliable manner, so that errors are practically excluded when the markers are identified and located.
In the present invention, a 3D volume data set is a data set of image data of a subject, from which different 3D images, which can represent different perspectives and sections of the subject, can be reconstructed.
In a version of the invention, the reference point, through which a back projection straight line extends, is the center of gravity of the marker. Using the center of gravity as the reference point has proven to be advantageous because it can be identified in a simple and defined manner for each detected marker. Therefore, it is not required to specifically specify how reference points are to be determined.
According to an embodiment of the invention, the detection of one or more markers that are imaged in a picked up or generated 2D projection is undertaken according to the following method steps. A 2D projection is divided into first surface elements. The picture element with the maximum or minimum light intensity is identified in each of these first surface elements. The local maximum and minimum of the light intensity in a second surface element of defined size is determined, which second surface element is positioned around the corresponding picture element, given an upward transgression or downward transgression of a first threshold value of the light intensity of such a picture element. A weighted difference is formed from the local maximum and minimum. The shape of a marker is developed when a second threshold value is exceeded by the weighted difference. Dependent on its shape, the marker is allocated to a marker type.
According to a version of the invention, when the shape of a marker is developed, the variance and/or the covariance and/or the correlation coefficients to adjacent picture elements in the plane of the 2D projection arc determined in an iterative processxe2x80x94proceeding from the respective maximum or minimum- and/or the number of the picture elements are determined. The principal inertial axes of a marker candidate are calculated when the detected values of the variance and/or of the covariance and/or of the correlation coefficients and/or the number of the picture elements are situated in a fixed range of values. The position of the principal inertial axes of the marker candidate relative to one another in the plane of the respective 2D projection are compared to the position of the principal inertial axes of markers of a known type relative to one another. The edge lines for the potential markers are formed when the relative position of the principal inertial axes substantially coincide.
The markers, particularly the shapes of markers, are thus reliably detected or identified in the 2D projections, so that their centers of gravity can be detected without problems.